Hey folks, I see a lot of outboard prop questions, but I am looking at buying an 89 Gulfstream with a Mercruiser 5.7 Alpha 1 Gen 2 sterndrive. I believe the gear ratio is 1.47. Any ideas on what size prop should be on this, the tack says 4600 at 22 knots and wot is 5100 at 33. Everything I have read tells me it should top out at around 4600 not 5100 so I am guessing it is a prop issue. Any advice would be appreciated, thanks.
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1989 Gulfstream Mercruiser 5.7
- Thread starter Altamira
- Start date
JeffN
Well-Known Member
Is the hub on the prop good? A slipping hub would cause the shaft to spin faster than the prop. Higher RPM less speed. If it is improper pitch IIRC the rule of thumb is an increase of 1" in pitch would decrease RPMs by 200. A decrease of 1" in pitch would increase RPM by 200. So you sound under pitched to me. Better check that though, there have been threads about pitch so a search should turn up some info and I would guess that I/O and outboards should be similar. What is on there now? Prop may not be of the proper diameter. Grady may be able to tell you what prop they used to ship that boat with. First I would find out what you have on there and then find someone who can tell you what you should have. Your Mercruiser tech would be a good place to start. Also somewhere on the motor there should be a tag with the correct RPM range for your motor. Should not be too hard to sort out.
JeffN
Well-Known Member
Nice hope she tests out OK. In the meantime try to give Grady a call and see what prop they suggested back when. Not too many here have I/Os we are a small but select group you know. What sort of shape is she in? The Gulfstream is a nice boat I would have been tempted but it came out after I purchased my Kingfish.
'84Offshore
Well-Known Member
Is that Gulfstream the 23' w/ 9' beam?
I have a 24' Offshore w/ a 5.7 w/ a 1.5:1 gear ratio turnin a 17" prop.
get 30mph @ 3600 RPM, wot ~4500 RPM going ~40MPH.
Hope that helps......
I have a 24' Offshore w/ a 5.7 w/ a 1.5:1 gear ratio turnin a 17" prop.
get 30mph @ 3600 RPM, wot ~4500 RPM going ~40MPH.
Hope that helps......
Offshore, yes it is the 9' beam. I'll be taking her for sea trials this Saturday again, not sure if the owner gave me the correct prop size. What does your boat weigh in at? I would love to get those numbers with this boat.
'84Offshore
Well-Known Member
I think the boat weighs ~4000lbs.
Is the 5.7 a vortec? It would have been upgaded.
Also, you might want to check the accuracy of the tach.
How was the speed measured.........GPS?
Is the 5.7 a vortec? It would have been upgaded.
Also, you might want to check the accuracy of the tach.
How was the speed measured.........GPS?
JeffN
Well-Known Member
You can go on the Grady website and access old catalogs for your year and the weights of the boats are listed. It is pretty cool to check out what was new and exciting back when too.
Edit - I just went on the Grady site and the 89 catalog lists your boat with a 260 HP I/O at 5000#. That is probably dry also so she is a big girl all up.
Edit - I just went on the Grady site and the 89 catalog lists your boat with a 260 HP I/O at 5000#. That is probably dry also so she is a big girl all up.
Jeff/Offshore,
Not sure if it is a vortec, how could I tell and what does vortec get you. Also I did question the tack when I was on the boat, it was a needle gauge and bounced a bit. During sea trials this Saturday I will have my mechanic check the tack, timing etc. The speed was on the chart plotter. I also checked with the guys at propgods and they suggested a 15 square enertia mercury prop. She is a beefy boat.
Not sure if it is a vortec, how could I tell and what does vortec get you. Also I did question the tack when I was on the boat, it was a needle gauge and bounced a bit. During sea trials this Saturday I will have my mechanic check the tack, timing etc. The speed was on the chart plotter. I also checked with the guys at propgods and they suggested a 15 square enertia mercury prop. She is a beefy boat.
'84Offshore
Well-Known Member
Check how many bolts are holding the intake manifold on.....8=Vortec, 12=non-Vortec.
Vortec heads will add 30-40HP to a 5.7L SBC, your mech should be able to tell.
With the Gulfstream being ~1000lbs heavier than my Offshore I would expect you to be turning a 15" prop....
Sounds like the speed was measured by GPS.......I would ask your mech to bring a tach for verification.
Should also perform a compression test........
It's a great boat BTW.........are you sure the outdrive is a Gen II?
Vortec heads will add 30-40HP to a 5.7L SBC, your mech should be able to tell.
With the Gulfstream being ~1000lbs heavier than my Offshore I would expect you to be turning a 15" prop....
Sounds like the speed was measured by GPS.......I would ask your mech to bring a tach for verification.
Should also perform a compression test........
It's a great boat BTW.........are you sure the outdrive is a Gen II?
Offshore, that's what the owner said - Gen 2. We'll find that out on Saturday also. I'm confused on the whole prop thing, I read to slow down rpm you increase pitch? I guess I have to find out for sure the gear ratio. Anything else to check on the sea trials, please let me know.
Offshore, that's what the owner said - Gen 2. We'll find that out on Saturday also. I'm confused on the whole prop thing, I read to slow down rpm you increase pitch? I guess I have to find out for sure the gear ratio. Anything else to check on the sea trials, please let me know.
JeffN
Well-Known Member
Altamira - are you local to the cape? Are you buying through a yard? Sent you a PM. If that engine is the vortech it would be sweet. I had one of those in a Suburban and It was very nice.
'84Offshore
Well-Known Member
Yes.......in order to Decrease RPM's you need to Increase prop pitch and visa versa...........
I'm just suggesting that given our weight differences and "similar" engines you will probably need to run a lower pitch pop than me.
I'm just suggesting that given our weight differences and "similar" engines you will probably need to run a lower pitch pop than me.
Jeff I pm'd you back, I am over the bridge, Bridgewater to be exact but the boat will be kept in Sandwich.
Offshore, thanks for the info, I guess I'll need to find a prop shop that will let me test out a few if that is determined to be the issue.
Thanks guys
Offshore, thanks for the info, I guess I'll need to find a prop shop that will let me test out a few if that is determined to be the issue.
Thanks guys